Prime Minister John Key has opened the door to reviving the so-called "waka jumping" law after NZ First MP Brendan Horan's expulsion from caucus.

Key told breakfast television this morning it was possible Parliament "might hold hands and look at this issue and decide once more to put something permanently in place" in the wake of the Horan incident.

"The issue is, it's really difficult to write the rules ... but if you in theory could write the rules then it's possible."

NZ First leader Winston Peters announced Horan's expulsion last week after allegations surrounding money from his dead mother's estate.

The NZ First board is expected to discuss his expulsion from the party tonight. Horan, who was elected off the NZ First list, has made it clear he intends to remain in Parliament as an independent MP.

Key said he did not know whether Peters had done the right thing because he was yet to produce the evidence against Horan.

That was the difficulty with any legislation designed to deal with such situations.

"What happens if Winston Peters decides this is all about politics, it's a bad look that someone's taking money from their dying mother so I'll just kick him out for political reasons then it's not proven. Then where does that leave us?"

The so-called "waka jumping law" was passed after former NZ First MPs, including now National MP Tau Henare, left the party so they could retain their ministerial portfolios after the 1996 National-NZ First coalition fell apart.

A former Alliance MP, the late Alamein Kopu also sparked widespread condemnation after she jumped ship to cross sides and support National.

ACT successfully used the waka jumping law to expel fraudster Donna Awatere Huata. But the legislation had a sunset clause and has since expired.

Key said there had been several recent cases which had highlighted the issue, including the case of former ACT MP David Garrett, who admitted stealing a dead child's identity, and the defection of United Future MP Gordon Copeland to become an independent.

"In my view [Mr Copeland] had no mandate to be there because Peter Dunne was the leader, he had effectively through his efforts in the party attracted the votes ... and had a certain proportional representation in Parliament. That was reduced. Same here for NZ First."